The invention relates to an AC plasma display panel.
Plasma display panels have many advantages compared to other displays currently used: they are flat, not subject to flickering, can be viewed in a wide angle and their brightness is comparable to the brightness of cathode ray tubes. In spite of the fact that the sequential excitation of pixels (picture elements) presents more difficulties than with cathode ray tubes (CRT), they may replace, in the future, such CRTs for the display of all kinds of pictures, more particularly in television receivers.
The AC plasma display panels are either of the coplanar type or of the matrix type.
It is recalled here the principle and structure of such plasma display panels. Both types of displays comprise two insulating plates, e.g. made of glass, separated by parallel partitions constituting for instance ribs of one plate. These plates form a sealed space containing a discharge gas such as a mixture of neon and xenon. The back or front plate is covered with phosphors. In case of color displays, each pixel comprises red, green and blue phosphors. In other words, each pixel comprises three cells, one for each color.
In a matrix type panel, a first set of parallel electrodes, called row electrodes, perpendicular to the ribs, are formed on the inner side of, for instance, the back plate. Each row electrode is associated to a cell. A second set of electrodes, called column electrodes, parallel to the ribs, are formed on the inner side of the other (front) plate. To each cell is associated one row electrode and one column electrode.
It should be noted here that, in an AC display, a dielectric layer covers at least one set of electrodes.
When a high voltage pulse is applied between the row electrode and the column electrode of the cell, an electric discharge is created within this cell. This electric discharge generates ultra-violet (UV) light which excites the phosphor of the addressed cell. This visible light, resulting from the excitation of the phosphor, is viewed through the transparent front plate. The electric discharge is maintained during a controlled duration by the application of an alternate voltage (AC) between the row electrode and the column electrode. The controlled duration corresponds to the amplitude of the corresponding color component to be displayed.
As the visible light is seen through the transparent front plate, it is preferable that the column electrodes be positioned at locations which are not facing cells. This goal is achieved if the column electrodes face the edges of the ribs constituting borders between pixels.
In a known device of this type, each column electrode has an axis which is coincident with the axis of the corresponding edge of the partition wall. It has been found that, with this embodiment, there is a risk that a discharge be produced on both sides of the partition wall and, therefore, two cells may be excited at the same time. Up to now, no satisfactory solution to this problem has been found. It is the reason why this kind of device has not been used in practice for a matrix type plasma display panel.
In a coplanar plasma display panel, a first set of parallel electrodes, called column electrodes, parallel to the ribs, are associated with the back plate. Each column electrode corresponds to a cell. It is usually disposed between two ribs under the phosphor layer. A second set of electrodes, called row electrodes, perpendicular to the ribs, are associated with the transparent front plate. Two row electrodes correspond to one cell. As these row electrodes are interposed between the phosphors and the face of the front plate to be seen by the viewer, they are usually transparent, for instance made of ITO (Indium-Tin Oxide).
In order to excite the phosphors of a cell, it is necessary to convert the gas of the cell into a plasma which generates ultra-violet (UV) light. For obtaining the plasma within a cell, an addressing pulse is applied between the column electrode of this cell and one row electrode. This pulse generates charges on the walls of the cell and these wall charges are seeds for a space discharge which is generated by the application of an AC voltage (for instance of a frequency comprised between 100 kHz and 500 kHz) between the two row electrodes of the cell.
The AC voltage is maintained during a controlled duration, called sustain period, which corresponds to the amplitude of the corresponding color component to be displayed. The row electrodes are generally called sustain electrodes.
The light transmission efficiency of transparent row electrodes cannot be 100%. Therefore, a part of the emitted light is masked by these electrodes. The transparency of these electrodes may be increased by reducing their thickness; it is also possible to reduce their width. But these reductions increase the electrical resistance of the electrodes and, a high resistance of electrodes is detrimental to the efficiency and simplicity of the control circuits of the display.
The invention solves the problems mentioned here above about matrix type displays and coplanar type displays. For coplanar type displays, the invention reduces the obstacles to the path of the light between the phosphors and the display face of the panel. For matrix type displays, the invention prevents the excitation of neighboring cells without any significant obstacle on the path of the light.
According to the invention, the electrodes on the front face are facing the edges of the corresponding partition walls and they comprise transparent tongues or protrusions towards the cells to be excited.
It will be appreciated that, for matrix type displays, the provision of transparent tongues does not reduce the light energy transmitted by the display and solves the problem of the non excitation of neighboring cells by imparting to the electric field of the cell to be excited, on one side of the partition, a value which is greater than the electric field on the other side of the partition, provided that, in this case, the electric field on the other side be below the excitation threshold.
For coplanar type displays, the invention provides a significant reduction of the obstacles on the path of the emitted light, the coplanar sustain electrodes being, of course, on the back face of the display.
In an embodiment, each column electrode is disposed closer from the side of the partition wall where is located the cell to be excited, than from the other side.
In that case, the column electrodes may be bands, for instance straight bands, having their axis shifted towards the side to be excited.
The transparent protrusions or tongues are for instance made of a thin film metal, such as ITO (Indium Tin Oxide).
As the column electrodes, facing the edges of walls, are positioned in locations where they do not decrease the visibility of cells, they can be realized with a low electric resistance. This is favorable to the efficiency and simplicity of the control circuits of the display.
The invention provides a plasma display panel comprising a back substrate, a first set of parallel electrodes associated to the back substrate, a front transparent substrate, a second set of electrodes associated to the front substrate, the electrodes of the second set having a direction which is transverse with respect to the direction of the electrodes of the first set, and partition walls which are situated between the back and the front substrates, and extend in the direction of the second set of electrodes. This display is characterized in that:
each electrode of the second set faces the edge of a corresponding partition wall, and
each electrode of the second set comprises, for each cell, a transparent protrusion extending towards the side of the partition wall corresponding to this cell.
In an embodiment each protrusion faces the phosphor(s) of the corresponding cell.
The length of the protrusions is, for example, a fraction of the width separating two partition walls.
In an embodiment, the axis of each electrode of the second set is shifted, with respect to the axis of the facing edge of the corresponding partition wall, towards the side of the cell to excite.
In an embodiment, each electrode of the second set is covered with black paint in the area in front of the edge of the w corresponding partition wall.
In an embodiment, where the plasma display panel is of the matrix type wherein the sustain AC voltage for each cell is provided between the corresponding electrodes of the first set and the second set, the panel comprises means for imparting to the electric field of the cell to be excited, on one side of the partition wall, a value which is greater than the value of the electric field on the other side of the partition walls, the electric field on this other side being below the excitation threshold.
In that case each protrusion may face a corresponding electrode of the first set.
According to another embodiment, the panel is of the coplanar type, wherein the electrodes of the first set are arranged by pairs, and the AC sustain voltage is provided between the electrodes of each pair; further, the protrusion is facing one electrode of the corresponding pair or the interval between the electrodes of the corresponding pair.
In this embodiment, the phosphors of the cells may be associated with the front substrate.